A New Twist on Soft Tooth Enamel

When World War II ended and the US troops came home, American popular culture began to pick up where it had left off. Among the more mundane, back-to-normal habits that some Amer­icans embraced was buying lots of lemons. Physicians had long claimed that lemon juice, rich in vitamin C, could help to fend off bouts of rheu­matism, colds, and other common maladies. But in the spring of 1947, dentists at the Mayo Clinic sounded the first of several sour notes to come on the practice. In a case study of 50 patients, the Mayo Clinic dentists observed that too much lemon juice in the diet could lead to bad teeth. The explanation: lemons are so highly acidic that, when consumed in excess, the juice corrodes tooth enamel. Sixty-three years later, the corrosive effects of lemon juice on tooth enamel are fairly well known across America. But still open to discussion is a related question that arises early in life. Do abnormally low acidic conditions in the tooth bud lead to poorly mineralized, soft enamel?
     In the August 6, 2010 issue of the Journal of Biological Chemistry (Vol­ume 285, Number 32), a team of Na­tional Institute of Dental and Cranio­facial Research (NIDCR)-supported scientists and colleagues offer an an­swer that is rooted in another recent discovery. Two years ago, the group showed for the first time that enamel-forming cells called amelo­blasts produce NBCe1, a protein known to transport bicarbonate and thus buf­fer pH levels in the microenvironment adjacent to the cell. As the authors then explained, pH levels normally vary during enamel formation, suggesting an obvious role and need for NBCe1. But the scientists needed to take the next step and establish that NBCe1 is essential for normal formation of dental enamel. In the current paper, the authors found in mouse studies that their suspicions were correct. Mice bred to produce no NBCe1 developed hy­po­mineralized enamel that was ex­tremely soft and fragile from the ameloblast’s inability to control the acidity levels in its mi­cro­environment.
     “The biophysical properties of the enamel are highly perturbed as a direct consequence of abnormal de­velopment due to the loss of NBCe1 function,” the authors concluded. “Thus, although it is widely appreciated that the acidity of ingested foods, drinks, or acidic salivary pH can affect enamel once fully formed teeth are exposed to these factors in the oral cavity, the results of this study demonstrate the importance of ‘internal’ acid-base homeostasis in the normal development of enamel in mammals.” Interestingly, the au­thors noted that this soft-enamel phenotype seems to exist in people born with a rare syndrome called proximal renal tubular acidosis. The condition is caused by alterations in the gene that encodes NBCe1.